1. Field of the Application
The present invention relates to the rolling of sheet metal billets and, more particularly, to methods for manufacturing sheet products of aluminum alloys, stainless steel and other metals with a mirror or dull surface finish.
2. Description of the Prior Art
There are widely known methods for manufacturing rolled sheet products, whereby a sheet billet is worked on both sides to produce a mirror or dull finish. The working is normally carried out by directing a jet of a bulk material at the surface of a sheet metal billet and is followed by chemical treatment and rolling. For example, a sheet metal billet is transferred from a hot-rolling mill to a shot blast machine where the surface of the billet is exposed to a blast of metal shot with particle sizes of 2 to 5 mm. The shot includes sharp-edged particles which partially remove the scale from the billet surface. The billet is then subjected to chemical treatment in alkaline baths and washed, whereupon it is etched in acid baths, again washed and dried. The billet thus treated is sent to a rolling mill.
The rolling is done in a single pass or in a number of passes, in which case each pass is followed by heat treatment of the billet and a removal of scale from its surface. For this purpose, the billet undergoes the above-mentioned operations of shot blasting, chemical treatment in alkaline and acid baths, washing and drying. At the final pass the billet is passivated in nitric acid solution. The cold-rolled product is then worked on a temper mill and cut into sheets. Each sheet is worked on grinding and polishing machines.
As a result, rolled sheets of stainless steel are given a mirror finish.
The application of the above method involves heavy expenses, keeping in mind that it requires much floorspace for auxiliary equipment and considerable amounts of chemical reagents and water. In addition, the process takes much time and demands skilled personnel who have to work in contact with aggressive media. The method under review is further disadvantageous in that the etching of sheet metal billets in acid solutions causes point corrosion which cannot be eliminated by subsequent treatment. Underetching or overetching of the surface of cold-rolled band is another cause of surface defects. However, the most serious disadvantage of the method lies in the necessity of repeatedly performing slow and laborious grinding and polishing operations with the use of special-purpose machines, which operations are in many cases ineffective because of surface flaws brought about by multiple etching operations.
There is commonly known a method for manufacturing bright lustre sheet of aluminum alloys, which is normally used for facing purposes. The method is carried out on cold-rolling mills with the use of oil. The sheet quality largely depends on the quality of the original hot-rolled band, which is the reason why the method does not necessarily yield good results. Besides, the use of pure oil is always fraught with fire hazards, which accounts for the fact that the cost of the fire extinguishing means may be as high as 15 percent of the cost of the entire cold-rolling mill, although even highly sophisticated fire prevention means cannot always preclude a blaze. The use of coolants of the oil-in-water type reduces fire hazards to a minimum, but tends to increase the amount of rejects.
The latter method for manufacturing aluminum alloy sheet does not make it possible to produce cold-rolled sheet with a dull surface onto which a paint coating could subsequently be applied. A dull surface is normally produced on a finished article. As a rule, such operations are carried out at metal-working facilities through the use of various techniques, including the working of articles with microspheres. A microsphere is to be understood as a solid hard spherical body of a diameter measured in microns. The use of microspheres calls for appropriate machinery which, in turn, requires some floorspace and personnel.
The known methods for producing bright lustre sheet necessitate the use of acids and alkalis for cleaning the surfaces of billets and thus can lead to air and water pollution.
The growing production of rolled sheet articles calls for a rolling method which would be more economical and less hazardous from the viewpoint of pollution than the existing methods. Such a method must also ensure the production of sheet products with polished surfaces on the cold-rolling mills. Finally, such a method must ensure a higher productivity and be carried out in such a way as to make the quality of finished products less dependent upon the surface quality of original hot-rolled billets.